In multi-stage motor vehicle automatic transmissions known from current practice shifting elements designed as wet-operating clutches or brakes are used between a transmission input shaft and a transmission output shaft of the automatic transmission for engaging various gear steps. For this, depending on the gear step desired, opening or closing of the shifting elements takes place. The pressure force required for this is usually applied for each shifting element by a hydraulically actuated clutch piston, which is supplied with hydraulic fluid by way of shifting element valves also known as pressure-reducing valves. These pressure-reducing valves are either themselves designed as proportional pressure-control valves or they are actuated by a hydraulic pilot control system such that the hydraulic pressure required for the pilot control is again set by a proportional pressure-control valve. In such a proportional pressure-control valve, as a function of an energizing current a magnetic force is produced, depending on which a certain, predictable working pressure is set at the valve. That pressure can be measured in an outlet area of the valve and is determined by the ratio between the magnetic force (action force) and a return force directed in opposition to the magnetic force (reaction force).
Proportional pressure-control valves common nowadays, for example the one disclosed in WO 2005/026858 A1, have two seat valves coupled in a hydraulic semi-bridge circuit, i.e. they have an inlet area and two outlet areas such that in terms of the flow a first seat valve is arranged between the inlet area and the first outlet area and a second seat valve is arranged between the first outlet area and the second outlet area. In this case the seat valves are designed, and their closing elements are coupled with one another, in such manner that in their end positions the closing elements close or open the seat valves in alternation.
To reduce the flow resistance and increase the dynamic controllability of a transmission shifting element controlled by the pressure-control valve, in WO 2005/026858 A1 it is proposed to insert between the first and second outlet areas a flow guiding device, specifically a stream deflector, which deflects a fluid flow from the first to the second seat valve by less than 30°.
From WO 2009/092488 A1 it is also known to provide such a flow guiding device with a plurality of duct areas in such manner that fluid flowing in the direction of the second seat valve has imparted to it a twisting motion, whereby the valve dynamics are improved and the valve leakage is reduced.
Furthermore, DE 100 34 959 A1 shows and describes a proportional pressure-control valve comprising a valve portion with inlet and outlet openings and at least one closing element for controlling an aperture, and a magnetic portion with a magnet core, a solenoid and a movable magnet armature. An actuator co-operates with the armature, the actuator actuating the ball-type closing element so that the hydraulically effective cross-section of an aperture is determined essentially by the aperture length, the aperture diameter and the diameter of the part of the actuator that penetrates into the aperture. In this proportional pressure-control valve the ratio of the aperture length to the aperture diameter should be <2.0.
This achieves better hydraulic properties of a proportional pressure-control valve, in particular a valve with optimized through-flow which, especially in the range of lower temperatures, i.e. when the viscosity of the hydraulic fluid is greater, shows substantially lower flow resistances. Correspondingly, the inlet geometry of the proportional pressure-control valve that determines the through-flow is optimally defined, with a ratio of aperture length to aperture diameter <2.0 and the through-flow determining aperture arranged in the inlet opening of the valve. Advantageously, the result is that the valve concerned has low flow losses, particularly at high oil viscosities, i.e. low temperatures. Moreover higher through-flow quantities and shorter valve response times are obtained, so that a proportional pressure-control valve of this design enables better dynamic values, with advantage.
Despite these design improvements, with these pressure-control valves known from the prior art, on the inlet or outlet side pressure fluctuations, for example produced by pump pressure fluctuations or slip-stick effects in the shifting elements of a vehicle transmission, can act almost without impediment on the closing elements of the seat valves or their operating position, and this makes it difficult to set a working pressure that can be determined at the valve.
In particular, the design principle of seat valves makes a certain inflow pressure dependence unavoidable in practice. This leads to an inlet pressure dependent inaccuracy in the adjustment of the position of the actuating element carrying the closing element of the valve. Especially in the low-load range of a powershift automatic transmission (for example during coasting downshifts) the inflow pressure dependence of such pressure-control valves can be perceived as particularly annoying. This is particularly the case when inflow pressure fluctuations occur in such a load condition, with consequent deviations from the nominal pressure which are amplified by clutch valve transmission ratios to the point that shifting behavior becomes unacceptable. Accordingly there is a need to optimize the behavior of pressure-control valves, particularly at low control pressures and pronounced inlet pressure drops, to the effect that as a result only minimal deviations from nominal pressure occur and the quality of shifts remains virtually uninfluenced by them.